Asynchronous motor



Sept. 1, 1925.

C. MACMILLAN ASYNCHRONOUS MOTCR Filed June 7, 1922 OOO.

Hiss. Atownec Patented Sept. 1, 1925.

UNITED STATES PATENT OFFICE.

CAMPBELL MACMTLLAN, or scHENEoTAnY, NEW Yoan, AssIGNoa 'ro GENERAL ELEC- TRIO COMPANY, A CORPORATION OF NEW YORK.

AsYNcHEoNoUs Moron.

applicati@ ined zum@ 7, f

TOUZ 107mm 'may concer/1L.'

Be it lrnown that I, CAMPBELL MAoMiL- LAN, a citizen ot the United States, residing at Schenectady, in the county ot Schenectady, State oi N ew York, have invented certain new and use'liul Improvements in Asynchronous Motors, ot which the following is a speciiication.

hly invention relates to alterating current dynamo electric machines and moreparticularly4 to the secondary members of alternating current motorsof the asynchronous type.

The principal object or" my invention is to provide amotor having` the desirable starting torque and running eiiiciency of the wound rotor type induction motor without the necessity ot providing the external starting resistance or complicated brush gear mechanism commonly used with such motors.

My invention contemplates improvements in the type ot induction motor rotor described in Gorges U. S. Patent 655,3a0. The most important novel feature ot the present invention consists in the arrangement ot the secondary winding so as to preserve cornplete electrical symmetry and at the same time avoid any increase'in reactanceover the usual external resistance connected secondary.

ln carrying my invention into effect I providea wound sccondaryconnected to a short circuiting device, whichl secondary is so wound that with the short circuit removed, rel atirely large currents are caused to'low in local'low inductance, high resistance' circuits in Vthe secondary windings, whereby high starting torque is obtained and when the windings of the secondary are short circuited the eiect produced is similar to the eiect produced by short circuiting the usual eX- ternal starting resistance of the ordinary Awound rotor induction motor whereby eilt cient running conditions are obtained. The rotor winding constituting the subject of the present application is especially applicable tor slow speed motors with large starting and reversing torque such, for example, as those used to drive the propellers of an electrically driven ship where it is desirable to keep the weight and space required for the motor equipment4 as low as possible. The rotor windingfconsists essentially of a normal delinite winding 'preferablyconnected 1922. serial No. 566,552.

toslip rings when the secondary forms the rotating element oit the motor as is usual. The rotor windings have unequal parallel paths in each phase. Villen the slip rings are open a circulating current is forced hackward through the low voltage circuits by the high voltage circuits, so that with a comparatively small net current per slot the current density in each conductor is considerably larger than usual due to the large currents flowing in opposite directions 4in the same slot the difference ot which gives the netV effective secondary current. When the rotor has been brought upto speed the slip rings are short circuited which causes the currents in the two parallel circuits per phase to flow in theV samel dir ction so that with this connection the elective slot current is the sum ot the currents flowing in each conductor.

The features of my invention which I believe Yto be novel and patentable will be pointed out in the claims appended hereto. The preferred construction and the theory of operationot a motor built in accordance with my invention will be described in connect-ion' with the accompanying drawing in which: ,Fig l illustrates a schematicv diagram ot the essential circuits ot the motor; Fig. 2 illustrates a portion ot the rotor cir cuit wiring diagram Figs. 3 and a illustrate the current distribution tor alternative ai rangements of the conductors per slot during the starting period; and Fig. 5 illustrates the distribution 'of current per slot during normal running conditions.l

' Referring first to Fig. l, 5 represents'the usual primary winding of a B-phase induction motor connected delta in this instance. The secondary winding is designated in general by the numeral 6 having three phases designated by the reference letters A, B and C, phase A being represented by `heavy lines, B by broken lines and C by light lines in order that said phases may be more easily traced in Fig. 2. The three phases or' the rotor are shown connected Y with the outer points connected to slip rings 7 mounted on the rotor shatt- 8. The slip rings 7 are arranged to be short circuited through stationary brushes 9 and the short circuiting switch l0; The diagram tor each rot-or phaseas illustrated in Fig. l may be taken to represent the arrangeinent ot 8 bars in two slots. A removable connector il constitutesy a noronly 116th of the 12R loss `for the same efiective current per slot as with the slip rings 9 open c-ircuited.

In operation, the external resistance motor is started with 'full line voltage or from an autotransformer or compensator with the slip rings open circuited. lVhen full speed is attained the slip ring circuits are closed by eontactors or switches. No external rheostats are required with this motor since the rotor winding gives the equivalent high resistance at starting, which resistance is reduced to normal when the slip rings are short circuited. The inductance of the rotor during thestarting operation is low due to the fact that conductors carrying current in opposite directions are placed side by side and this arrangement is continued in the end connections so far as is practicable.

Dueto the large energy loss in the roto. of my improved motor at starting, it be comes important to design the rotor so as to withstand and dissipate the heat generated therein. The insulation 17 should be capable of withstanding high temperatures and should also be a good heat conductor. It may often be desirable to use forced ventilation during'the starting period. It may be stated, however, that if the insulation 17 should burn through, the resulting short eireuit between bars would simply reduce the available starting torque and would not interfere with the normal operation of the motor.

The purpose or' theremovable connectors ll is so that this cross connection may be opened up in order to use the rotor with an external resistance for starting purposes in the ordinary way should it become desirable. lith the connectors 1l removed, the motor may be started up by the use o the ordinary external resistance connected to the slip rings and when the rotor is up to speed, the slip rings may be short eircuited in the usual way for normal operation. As thus used, the current distribution per slot would be unequal. The current in the parali lel and series coils would be in the ratio of two-thirds to four-thirds, giving a slight increase in the energy dissipation during normal operation. In any case where it becomes desirable to operate the motor in this manner the full eiiiciency` of uniform current density may be obtained by making the sections of the series and multiple bars proportional to the current which they will be called upon to carry. By the full efficiency ot' uniform current density, I refer to the well known tact that if a specified total current is carried in the specific number of multiple circuits of equal length with a specified total cross sectional area of conductor, that distribution of current, which gives minimum resistance losses is obtained when the current densities in the `various multiple circuits' are equal. There is no imperative reason'why these bars need have the same crosssectional dimensions. Other relations between these dimensions furnish a simple means for obtaining minor variations of the equivalent high resistance as desired. lth` the connectors ll removed, the motor may also b'e run inverted; that is to say, with the stator shortcircuited and the generator lines connected to the brush leads. Since the connectors 1l are arranged in the end connection they may be removed without dismantling the rotor. f

The winding connections for motors built in accordance with this invention are `not applicable to all combinations of poles and circuits but can generally be applied where the number of circuits is not very small. There must always be at least two conductors per slot. I therefore do not wish to be limited to the particular arrangement or' the circuits described, but intended to cover in the appended claims all modifications coming fairly within the scope of my invention. For example, instead of the unit group ot 8 conductors selected for description on account of its relative simplicity, other unit groups may be used consisting of numbers having suitable factors. Thus in a unit group or' 24 conductors in two circuits ot l2 coils each, these l2 coils might be segregated in sub-groups of which` the iirst would have 3 multiple 4 series and the second i multiple 3 series. The result is in every case to give an eifective current per slot proportional to the difference in current per circuit. The energy dissipating et'- iect, on the other hand, remains proportional to the sum of these currents, the tot-all losses in thetwo cases being proportional to the squares of these values. `-The ratio ot current per circuit is obtained as the reciprocal of the number of multiple circuits in the respective groups.

What I claim as new and desire to secure by Letters Patent of the United States, is:

l. In an alternating current motor, the combination with a ield portion, of an armature therefor having a multiphase winding comprising an even number of divisions for each phase, each of said divisions comprising a pair of parallel coils connected in series with a longer coil, the parallel coils of one division and the longer coil of another division being connected in local short circuits and means for interconnecting said phases to form a closed, electrically symmetrical phase wound winding.

2. In an alternating current induction motor, a secondary member having a. multiphase winding comprising an even number of equal divisions, each division having two relatively short coils in multiple and a longer coil in series, said divisions being interconnected so as t0 form av plurality of local short circuits each comprising the tivo multiple coils ot one division in series with the longer coil of a difl'erent division and means tor changing the connections olI said phases to vary the ei'llective resistance oi: said secondary.

3. In an alternating current motor the combination with a lield portion, of a armature therefor having` a nmltiphase Winding and having an even number ot equal divisions, unequal portions of di'lierent divisions being connected in local short circuits and means for connecting said dililerent divisions in parallel.

4l. In an alternating current motor, the

' combination with a primary, oft a secondary therefor having a multiphase Winding provided With a pair ot equal circuits per phase, unequal portions ot each pair being connected in voltage opposition and means for connecting said pairs ol circuits in multiple.

5. A secondary men'iber 'tor a motor or the induction type comprising an electrically symmetrical niultiphase winding situated in slots, said Winding comprising an even number of equal divisionH having' nnequal portions oit dil'l'erent divisions interconnected to form low inductance short circuits situated in t-he same slots and means for connecting said interconnected divisions in parallel.

An induction motor secondaryv ot the phase Wound type having unequal coil tions connected in opposition for producing a high internal resistance for starting` purposes and having means lor connecting said coil sections in parallel to decrease the internal resistance of the secondary Afor runs ning purposes, characterized by the tact that the opposing coil sectionsin the starting connection have an unequal number of circuits and are distributed in slots in such a Way that the ellective starting current per slot is the diilierence between the currents carried by an equal number ol opposing circuits i'. An induction motor secondary ot the phase Wound type having unequal coil sections connected in opposition Ator producing a high internal resistance for starting purposes and having means for connecting said coil sections in parallel to decrease the internal' resistance olf the secondary -for ruin ning purposes, characterized by the tact that the opposing coil sections in the starting connection have an unequal number ot coils in series and are distributed in slots in such a way that the e'tiect-ive starting current per slot is the dill'erence between the currents carried by an equal number olf opposing circuits and the effective running current per slot is the sum of the currents carried by said circuits.

8. in induction motor comprising cooperating primary and secondary members, said secondaryv member being phase Wound and oit the type having connections to convert the same Yfrom a loiv inductancej high internal resistance secondary to a lonv internal resistance secondary, the high internal resistance ei'lect being possible by reason ot connections between electrically unequal portions ot nornially parallel circuits in said secondary windinga characterized by a secon dary coil arrangement such that complete electrical symmetry as regards magnetic disn tribut-ion `is retained whereby the motor may be used .inverted Without modification ot 'the linx wave iorm` of linx distribution when the connections between the electrically unequal port-ions of the normally parallel circuits are removed.

ln Witness whereof, l have hereunto set my hand this 6th day of June, 1922.

CAMPBELL MACMILLAN. 

